In an electric power steering system for a vehicle, a steering assist motor is driven so as to obtain a preferred steering assist force in accordance with a steering torque given to a steering wheel by a driver and a vehicle speed.
The steering assist motor is driven by a motor drive circuit which is incorporated in an electronic control unit (hereinafter, referred to as ECU). The motor drive circuit controls large power of the order of 500 W to 2000 W when it drives the steering assist motor.
As this occurs, the motor drive circuit generates heat, in order to prevent erroneous operations or failure of the ECU due to the generated heat, the motor drive circuit is mounted on a circuit board having good heat conduction properties. As is shown in FIG. 10(a), for example, the motor drive circuit is mounted on a metallic circuit board in which one conductor layer 91 made of copper and one insulation layer 92 made of resin are formed one on top of the other on a metallic substrate 93 (a heat sink) made of aluminum.
Since electronic components can only be mounted on one side (specifically, on the conductor layer 91) of the circuit board shown in FIG. 10(a), an area occupied by this circuit board within the ECU becomes large. Then, as one of methods for reducing the substrate area, a method is considered in which a circuit board is fabricated with a multiplicity of layers, as is shown in FIG. 10(b). A ceramic multilayer substrate shown in FIG. 10(b) is such that copper conductor layers 91 and ceramic insulation layers 94 are laminated into a multilayer construction, which is then affixed to an aluminum metallic substrate 93 (a heat sink) with an adhesive 95. By adopting the multilayer construction, since the length of circuit conductors (the length of wirings) can be shortened, the inductance thereof can be suppressed to a lower level, thereby making it possible to suppress the generation of switching noise.
In relation to the invention of this patent application, the following related-art techniques are known. Patent Document 1 discloses an electric power steering system which includes a circuit case which is assembled to a heat sink while covering a power substrate including a switching element and in which the heat sink is assembled to a speed reduction mechanism. Patent Document 2 discloses an electric power steering system in which a power circuit board and a control circuit board are installed to be laminated one on top of the other in a circuit casing and the power circuit board is assembled closely to a bracket of a motor. These constructions eliminate wiring harnesses and connectors so as to reduce radiation noise. Further, Patent Document 3 discloses a configuration in which a substrate including a motor drive circuit and a control circuit is mounted on a motor case to thereby stabilize the operations of the circuits.
Patent Document 1: JP-A-2002-120739
Patent Document 2: JP-A-2002-345211
Patent Document 3: WO99/16654